The present invention relates to a visual optical device such as a telescope or binoculars that has an objective lens, an erecting system and eyepiece.
A visual optical device that includes an objective lens having a positive refractive power and an eyepiece having a positive refractive power requires an erecting system that erects an inverted real image formed by the objective lens when the device is used for a terrestrial observation.
Therefore, a visual optical device has the erecting system such as a pentagonal roof prism or Porro prisms located between the objective lens and the eyepiece. An inverted real image formed by the objective lens is erected by the erecting system, and then the eyepiece magnifies the erected real image for an observation by a naked eye.
Since manufacturing of roof surfaces of a pentagonal roof prism requires very high processing accuracy, a manufacturing cost becomes higher, which raises the total cost of the visual optical device. Therefore, the Porro prism erecting system is preferable to be used in the visual optical device.
FIG. 11 is a developed diagram of an optical system of a conventional visual optical device. The optical system 100 of this device consists of an objective lens 110, an erecting system 130 and an eyepiece 120, arranged in this order from an object side.
The erecting system 130 is a Porro prism erecting system that is provided with a first Porro prism and a second Porro prism. Each of the Porro prisms is a 45-90-45xc2x0 reflecting prism whose reflecting surfaces form a right angle for reflecting the light beam through a total angle of 180xc2x0. The four reflecting surfaces of the Porro prism erecting system can be replaced with mirrors. In the specification, a Porro type erecting system means both of the Porro prism erecting system and the combination of alternative mirrors.
It should be noted that the first and second Porro prisms are shown as boxes 131 and 132 illustrated by dotted lines in FIG. 11. The boxes 131 and 132 represent spaces occupied by the first and second Porro prisms, respectively.
Object light incident on the optical system 100 from the left-hand side in FIG. 11 is converged by the objective lens 110 to form a real image of an object. Since the light is reflected by four times through the first and second Porro prisms, an erected real image is formed on a field stop 140. The eyepiece 120 magnifies the erected real image.
The maximum incident angle xcfx89 is determined by the following equation:
xcfx89=tanxe2x88x921(y/fo)
where y is a radius of aperture of the field stop 140 and fo is a focal length of the objective lens 110. A real field of view is twice the maximum incident angle, i.e., 2xcfx89.
Since observing region becomes broader as the real field of view 2xcfx89 increases, it is preferable to increase the real field of view. The equation teaches two ways to increase the real field of view 2xcfx89. A first way is to decrease the focal length fo of the objective lens 110. A second way is to increase the radius of aperture y of the field stop 140.
However, if the focal length fo of the objective lens 110 becomes shorter without changing the radius of aperture y, the distance between the objective lens 110 and the field stop 140 becomes too short to locate the erecting system 130, as shown in FIG. 12.
On the other hand, if the radius of aperture y of the field stop 140 becomes larger without changing the space for the erecting system 130, the diameter of the eyepiece must be large as shown in FIG. 13. The erecting system 130 also becomes larger, which increases the total size of the device.
It is therefore an object of the present invention to provide a visual optical device such as a telescope and binoculars capable of increasing the real field of view with employing a Porro type erecting system and keeping a compact total size.
For the above object, according to the present invention, there is provided an improved telescope, which includes a Porro type erecting system having first, second, third and fourth reflecting surfaces that are arranged from an object side, an objective lens that is located between the second and third reflecting surfaces, and an eyepiece that is located between the fourth reflecting surface and an eye point. The eye point means a position of an exit pupil of the telescope.
The first and second reflecting surfaces and the third and fourth reflecting surfaces are faced to each other at a right angle, respectively. Further, an intersection line between extended surfaces of the first and second reflecting surfaces is perpendicular to an intersection line between extended surfaces of the third and fourth reflecting surfaces at twisted positions. The objective lens has a positive refractive power to converge object light that is reflected by the first and second reflecting surfaces for forming an image and the eyepiece has a positive refractive power to magnify the image formed by the objective lens.
With this construction, since the objective lens is located between the second and third reflecting surfaces, the distance between the objective lens and a field stop can be shorter than the conventional optical system, which enables to decrease the focal length of the objective lens without narrowing the space for the erecting system. Therefore, a real field of view can be larger with employing the Porro type erecting system and keeping a compact total size.
The first and second reflecting surfaces may be formed on separate mirrors or formed as back-surfaces of a right-angle prism. When the first and second reflecting surfaces are formed as back-surfaces of the same prism, adjustment between these reflecting surfaces becomes unnecessary, which eases the assembling of the reflecting surfaces onto the telescope.
In the same manner, the third and fourth reflecting surfaces may be formed on separate mirrors or formed as back-surfaces of a right-angle prism. If the first and second reflecting surfaces, and the third and fourth reflecting surfaces are formed as right angle prisms, respectively, these reflecting surfaces constitute a type I Porro prism erecting system.
According to an another aspect of the present invention, there is provided a telescope, which includes: a Porro type erecting system whose first reflecting surface is formed on a retractable mirror that can swing between a working position where the first and second reflecting surfaces are perpendicular and a retracted position where the free end of the retractable mirror moves close to the second reflecting surface, an objective lens that is located between the first reflecting surface and the fourth reflecting surface, and an eyepiece that is located between the fourth reflecting surface and an eye point.
With this construction, since the objective lens is located between the first reflecting surface and the fourth reflecting surface, the optical system of the telescope becomes thinner when the retractable mirror is in the retracted position.
Further, an angle adjusting mechanism may be added to the retracting mechanism for the retractable mirror in order to finely adjust the direction of the object light reflected by the retractable mirror. It is preferable that an incident optical axis, which is an optical axis of the objective lens bent by the reflecting surfaces between the object and the objective lens, are parallel to an exit optical axis, which is an optical axis of the eyepiece extended toward the eye point.
Binoculars of the invention is provided with the above described telescopes in a pair. Each of the telescopes includes the Porro type erecting system, the objective lens located between the second and third reflecting surfaces, and an eyepiece located between the fourth reflecting surface and an eye point.
The first reflecting surfaces of the pair of telescopes may be formed on a common mirror. Further, the first reflecting surfaces may be formed on the retractable mirrors. In the specific case, the first reflecting surfaces are formed on a common retractable mirror.
The second reflecting surfaces are preferably adjacent to each other in the same plane. Further, the second reflecting surfaces may be formed on a common mirror.